1. Field of the Invention
This invention relates to testing devices that perform testing on multistage multi-branch optical networks with respect to fault time, fault line and fault distance. This application is based on patent application No. Hei 9-161396 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
FIG. 7 shows an example of a testing device for a multi-branch optical network. This testing device performs fault isolation test on an optical network of 8-branch type provided in 1.31/1.55 .mu.m wavelength multiplex transmission system. In FIG. 7, an OTDR measurement device 1 (where "OTDR" is an abbreviation for "Optical Time Domain Reflectometer") generates test light (having 1.6 .mu.m band), which is incoming to an optical line 3 via a coupler 2. Then, the test light is subjected to branching by a star coupler 4, from which test beams are distributed to "branch" optical fibers whose numbers range from No. 1 to No. 8. Herein, filters 41 to 48 are respectively provided on the optical fibers No. 1 to No. 8 prior to their ONUs (where "ONU" is an abbreviation for "Optical Network Unit", which is an optical subscriber network unit). Each of the filters 41 to 48 has a specific band-pass characteristic that passes only a light signal corresponding to each of the ONUs while reflecting the test beam. Therefore, each of the test beams that progress through the optical fibers No. 1 to No. 8 respectively is reflected by each of the filters 41 to 48. Thus, a reflection beam given from each filter is transmitted backwardly on each optical fiber. Reflection beams, transmitted backwardly on the optical fibers No. 1 to No. 8, are subjected to wave mixing by the star coupler 4. As a result, mixed light is transmitted via the coupler 2 to the OTDR measurement device 1 as response light. So, the OTDR measurement device 1 performs analysis of the response light.
FIG. 8 shows an example of a waveform of the response light, which is supplied to the OTDR measurement device 1. This waveform corresponds to a time-series record of the waveform of the response light, which is supplied to the OTDR measurement device 1 via the coupler 2. In FIG. 8, a horizontal axis is given by multiplying a time axis by a transmission speed of the response light. That is, the horizontal axis of FIG. 8 represents a length of the optical fiber through which the response light propagates. So, the waveform of the response light is plotted in accordance with such a horizontal axis. By the way, the reponse light corresponds to a result of wave mixing of the reflection beams reflected by the filters 41 to 48 respectively. These filters are located at different positions on the optical fibers, in other words, these filters are located with different distances from the OTDR measurement device 1. For this reason, the reflection beams from the filters 41 to 48, which are measured by the OTDR measurement device 1, are not overlapped with each other on the time axis. So, they are measured as separate ones. A leftmost wave portion "R" of the waveform shown in FIG. 8 represents a reflection beam given from the star coupler 4. In FIG. 8, eight wave portions, which follow the leftmost wave portion R, are arranged from the left to the right. The eight wave portions correspond to eight reflection beams, which are transmitted backwardly on the optical fibers No. 1 to No. 8 respectively and are supplied to the OTDR measurement device 1.
FIG. 9A and FIG. 9B each show a set of three wave portions corresponding to reflection beams of the optical fibers No. 6 to No. 8, which are extracted from reflection beams measured by the OTDR measurement device 1. Herein, the three wave portions are magnified in scale. FIG. 9A shows an example of the three wave portions with respect to the case where fault occurs on none of the optical fibers, while FIG. 9B shows another example of the three wave portions with respect to the fault-simulated case where a bending loss of 3 dB is intentionally imparted to the optical fiber No. 7. It can be observed from FIG. 9A and FIG. 9B that intensity of the reflection beam is lowered due to occurrence of the simulated fault.
Thus, the testing device of FIG. 7 is capable of performing fault detection with respect to fault that occurs on the optical fiber by analyzing the intensity of each of the reflection beams included in the response light supplied to the OTDR measurement device 1. Incidentally, such a technology is disclosed by the paper B-846 entitled "1.6 .mu.m-band Fault Isolation Technique for Passive Double Star Networks", which is issued in 1994 autumn meeting of the Institute of Electronics, Information and Communication Engineers of Japan, for example.
The fault isolation test system of the multi-branch optical network, which is described above, is capable of performing fault detection only by specifying an optical line having a fault. However, this system is incapable of detecting a distance for a fault point, in other words, this system is incapable of detecting a location of a fault point. In addition, the aforementioned system provides requirement that different distances should be set from the coupler 2 to the filters 41 to 48 respectively. To achieve such a requirement, it is necessary to elongate the length(s) of the optical fiber(s) to some extent. As a result, the aforementioned system may introduce an increase of the cost for constructing the transmission system corresponding to the optical fibers. So, it is difficult to put the aforementioned system to practical use.